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Will sea levels rise sharply? Drilling could tell

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The 3-inch hole drilled here is a mere pinprick on a Texas-sized floating ice shelf. But the ancient sediments scientists have extracted through the tiny opening could eventually reveal just how much global warming it would take to raise sea levels by a few feet or more, and wipe out coastlines around the world.

Over a period of two months, crews drilled 4,200 feet to pull up tubes of sediment that reveal layers of history going back 13 million years. The sediment should tell scientists when, and how quickly, the Ross Ice Shelf has expanded and retreated over the eons as Earth's climate has warmed and cooled.

Experts fear that greenhouse gases generated with the advent of the Industrial Revolution are influencing climate on top of natural cycles. If that's the case, expect an accelerated thaw on Antarctica, a landmass bigger than the continental United States and covered in ice that averages a mile thick.

"If you want to understand how sea level is going to increase as the planet warms up, this is the perfect place," says Tom Wagner, a program adviser with the National Science Foundation, which helped fund the $30 million international Antarctic Drilling Project, known as ANDRILL.

Scientists targeted this particular spot because it's part of the Ross Ice Shelf, which holds back two mammoth pieces of ice that together represent 90 percent of the world's fresh water: the East Antarctic and West Antarctic ice sheets.

Melting ice shelves like Ross won't raise sea levels because they're already in the water. But their disappearance would accelerate the movement and melting of the ice sheets behind them.

The East Antarctic sheet is much larger but also considered much more stable. The West sheet, however, has already shown signs of melting.

Ice sheets can melt rapidly in geological terms — over a few centuries or even decades, says project co-chair Tim Naish. He says the West Antarctic sheet could raise sea levels by 6 to 9 feet "relatively quickly."

That's why scientists are so keen to see how, and under what temperatures, the Ross Ice Shelf has changed over time.

On the other side of the continent, an ice shelf the size of Rhode Island broke up in a few weeks in 2002. The Larsen B ice shelf was along the Antarctic Peninsula, a point close to South America and the fastest-warming region in the world, according to scientists.

Naish says that when he looks at the Ross sediment samples, he starts "thinking of collapse, not just retreat but collapse. And collapse along the lines of the Larsen Ice Shelf. That doesn't mean the whole ice shelf would collapse, but a sector of it could."

Crews drilled day and night at the ANDRILL base camp for two months beginning last November, pulling up cores of sediment every half hour or so and shipping them to nearby McMurdo Station, the main U.S. base on Antarctica.

At McMurdo, a feeding frenzy of experts descended on each day's bounty, hoping to claim a few slivers of sediment for their climate research.

"This is the engine room of global climate, yet we really don’t know at a fundamental level how it works," says Naish, a geology researcher at Victoria University in New Zealand.

Scientists don't expect definitive answers from their work for several years. But a preliminary analysis of the sediment shows that the Ross Ice Shelf retreated and expanded at least 60 times over the past 13 million years, rapid swings according to Naish.

"So that should ring alarm bells," he says. "It was quite dynamic."

Moreover, those swings occurred when levels of carbon dioxide, the key greenhouse gas tied to warming, were 30 percent lower than today.

It's not just rising sea levels that worry scientists. A rush of fresh water into the ocean from a  melting ice sheet could destabilize ocean currents, altering the marine ecosystem and possibly even the weather.

"If you very abruptly put a lot of melt water into the ocean you change the salinity of the ocean, you affect the whole productivity of the Southern Ocean, which is key to the whole global food chain," says Naish.

Salt water is denser than fresh water and sinks in a motion that creates currents. Melt water, however, dissipates instead of sinking and a huge amount could slow currents and reduce the amount of heat transported toward Europe, says Lionel Carter, another project scientist and a marine geologist at the University of Wellington in New Zealand.

With so much at stake, the scientists are treading carefully. In fact, they plan to drill several more holes to get a better history of the vast Ross Ice Shelf. The next drilling, set for 2008 at a nearby location, will extract sediment that goes back an additional 4 million to 5 million years.

After that, the drilling project could expand to other ice shelves around the white continent.

For Naish, Antarctica's ice shelves are "the early warning systems" of climate change that can alert mankind to potential disaster.